1. Field of the Invention
The present invention relates to a magnetic characteristics measuring method and system, and more particularly to a magnetic characteristics measuring method and system of high accuracy which uses an audio A/D converter and which is combined with a signal-processing technique for extracting a weak magnet signal through fundamental-wave extraction, fast Fourier transformation, and statistical processing (median calculation).
2. Description of the Related Art
Many electronic devices and functional materials, including magnetic recording materials, utilize unique magnetic characteristics. Nowadays, in consideration of environmental and energy concerns, there have been actively developed materials for high speed processing, energy-saving drive, and miniaturization. In a stage where a functional material is actually developed, the processes of “composition” and “evaluation” are repeated again and again. In this regard, an evaluation apparatus having improved performance has been demanded.
A magnetic measurement apparatus of high accuracy which utilizes a superconducting quantum interference device (SQUID) has been known as an apparatus for evaluating magnetic characteristics (see A. D. Hibbs et al., Rev. Sci, Instrum. 65 (1994) pp. 2644-2652). The superconducting quantum interference device, which includes a superconducting ring coupled to one or two Josephson junctions, is suitable for applications such as a high-sensitivity magnetometer, a near magnetic field antenna, and measurement of a very small current or voltage. This magnetic measurement apparatus can perform automatic measurement by means of sequence control under conditions of a wide temperature range (1.7 to 400K) and a strong magnetic field (±7 T).
FIG. 7 is a diagram showing a conventionally known SQUID circuit. As shown in the drawing, the maximum value Ic of current flowing from a DC power supply DC to a dc SQUID changes periodically with an external magnetic flux Φ. Since current is supplied from an AC power supply AC to a feedback coil, the output voltage V of the dc SQUID changes at a period Φ0. The modulated magnetic flux creates a modulated magnetic field in cooperation with the AC power supply, and tunes the circuit to an optimal low-noise state. A change in magnetic flux detected by a pickup coil (signal coil) is taken out, as an electrical signal, by use of the SQUID, which functions as a transformer. The change in magnetic flux detected by the pickup coil is reflected as a change in the external magnetic flux Φ. The principle of the fluxmeter is such that a change in the external magnetic flux is measured in the form of a change in voltage, by use of a portion of a change in the output voltage. This voltage change is output to the outside via a preamplifier and an integrator. A feedback circuit has a function of decreasing a change in the SQUID voltage (caused by a change in magnetic flux) to zero (resets the reference point), to thereby maintain the linearity between change in magnetic flux and change in voltage.
FIGS. 8A to 8C are diagrams showing three different systems using the conventionally known magnetic measurement apparatus. FIG. 8A shows a DC-mode measurement system; FIG. 8B shows a VCM-mode measurement system; and FIG. 8C shows an AC-mode measurement system. In the DC-mode system designed to measure the magnetic flux (DC magnetization) of a sample, the sample is placed in a superconducting coil for detection, the magnetic flux of the sample is measured, and a signal indicative of the measured magnetic flux is output via a SQUID probe. This system has a drawback of being weak against noise. In the VCM (vibrating coil magnetometer)-mode system, the DC magnetic susceptibility is measured through detection of the amplitude of the AC SQUID voltage at the time when the superconducting coil for detection is vibrated by means of an actuator and a piezoelectric element. This system has drawbacks in that the resonance condition of the piezoelectric element changes with temperature, maintaining a consistent vibration condition is difficult, and the output signal includes noise stemming from mechanical vibration. In the AC-mode system, a sample is placed in an alternating magnetic field generated by an AC coil, and the AC magnetic susceptibility is measured through detection of the AC signal amplitude of the SQUID voltage. As will be described later, the present invention enables magnetic measurement to be performed by use of a SQUID even in a situation where the magnetic shield is incomplete, through an improvement on the above-described AC-mode measurement system; in particular, an improvement on the signal processing technique for an electric signal from a SQUID controller.
FIG. 9 is a block diagram showing a conventionally known food inspection apparatus to which the above-described magnetic measurement apparatus is applied (see Japanese Patent Application Laid-Open (kokai) No. 2004-28955). In the illustrated food inspection apparatus, a coil is provided to surround a cryogenic insulating container in which a SQUID magnetic sensor is accommodated. The cryogenic insulating container and the coil are covered with a permalloy magnetic shield so as to shield them against external magnetic noise. A belt conveyer conveys a food (measurement sample) through an opening of the magnetic shield to a region inside the coil and just under the SQUID magnetic sensor.
The coil is connected to an AC oscillator, and generates an alternating magnetic field. The output of the SQUID magnetic sensor is fed via a sensor controller to a lock-in amplifier, at which the output undergoes phase detection and amplification, and is then output to a recorder. The signal from the sensor controller itself is also fed to the recorder via a filter, at which high-frequency components are removed from the signal. Thus, the output of the lock-in amplifier and the output of the filter are recorded. Further, the output of the lock-in amplifier and the output of the filter are also fed to a personal computer, which determines whether or not the food product contains a feebly-magnetic foreign substance. The signal output from the AC oscillator is also used as a reference voltage signal for the lock-in amplifier.
The lock-in amplifier has a function of selectively detecting a signal synchronized with the applied alternating magnetic field. For example, when an alternating magnetic field of 100 Hz is applied, among the components output from the SQUID magnetic sensor, only a signal component in a very narrow band around 100 Hz can be extracted (see “Example” to be described later).
Although apparatuses using such a lock-in amplifier have been used for more 10 years since the start of their global spread, their measurement accuracy has not been improved. In the case of such a conventional apparatus, improving measurement accuracy is difficult unless a sample to be measured produces a large signal or measurement is performed within a magnetic shield. Further, since the conventional apparatus using a lock-in amplifier is not designed to visually monitor the electronic signal in real time, it is difficult to cope with problems such as a large time delay of the detection signal in relation to the reference voltage signal, which occurs due to eddy current, and harmonic analysis cannot be performed easily.
Software of the conventional SQUID magnetic measurement apparatus is not designed with the assumption that a pressure-generation device formed of a metal is inserted into the apparatus. In actuality, in order to correct for an influence of eddy current flowing within a metallic component necessarily placed in the apparatus, basic data for phase adjustment are previously obtained by use of a standard sample (stored in a hard disk drive within a PC), and the data are used as a basis for correcting for the influence of the eddy current. However, when a metallic pressure cell having a large capacity is inserted into the apparatus for the purpose of, for example, a pressure experiment, the above-mentioned basic data become meaningless. The influence of eddy current generated upon use of a metallic cell is remarkable, and the phase shift reaches 90 degrees at 1 kHz, which indicates that proper measures for phase adjustment must be taken.
AC magnetic susceptibility can be decomposed into a component which follows an alternating magnetic field and a component which has a 90-degree phase shift and which represents energy loss. In general, such decomposition is performed by means of complex Fourier transformation; the former is called a “real component,” and the latter is called an “imaginary component.” For actual evaluation of magnetic characteristics, in many cases, evaluation of the real component is sufficient; however, in the case where more detailed physical information is required, attention must be paid to the imaginary component. The imaginary component reflects energy loss, and in the case where the magnitude of a magnetic moment attenuates with time, the size of an energy barrier which causes such a phenomenon (which serves as an index of stability in term of realization of a metastable electronic state) can be estimated from the frequency dependency of the peak position of the imaginary component. Further, since the imaginary component does not include a paramagnetic signal attributable to magnetic ions of an impurity, use of the imaginary component is effective for evaluation of the magnetic transition temperature of a magnetic material of low purity. As described above, the imaginary component becomes an important physical quantity when a magnetic relaxation phenomenon (essential for studies on nano-size magnetic materials) is examined thoroughly or when the magnetic transition temperature of a subject substance is evaluated.
Moreover, since the conventional SQUID magnetic measurement apparatus is designed with importance placed on DC measurement, the apparatus does not enable use of a SQUID in a high-frequency alternating magnetic field. Heretofore, apparatuses using a SQUID can measure AC magnetic susceptibility up to about 1.5 kHz. When a relaxation phenomenon of a magnetic behavior is studied on the basis of AC magnetic susceptibility, the wider the frequency range of an alternating magnetic field (the importance of AC magnetic susceptibility increases stepwise with every order of magnitude of frequency-range expansion), the more clear the mechanism of expressing the magnetic behavior. However, the upper limit of the frequency range of the above-mentioned apparatus is 1 kHz, and studies have not yet been performed on high accuracy measurement of the magnetic behavior in a high frequency band. The apparatuses which do not use a SQUID (for example, those which utilize simple mutual induction) are insufficient in terms of accuracy, although they can perform measurement up to 10 kHz. As described above, expansion of the frequency range is scientifically important, because of its potential value, even if the expansion is only one order of magnitude.